In an image forming apparatus based on electrophotographic method, a one-component developing system and a two-component developing system have been known, wherein the one-component developing system uses only toner as a developer in the development method for electrostatic latent image formed on an image carrying member, whereas the two-component developing system uses both toner and carrier.
In the one-component developing system, a toner carrying member and a regulating plate pressed against the toner carrying member are generally used. Control of film thickness is performed by pressing the toner on the toner carrying member by the regulating plate, thereby forming a thin toner layer charged with a predetermined amount of charge. An electrostatic latent image on an image carrying member is developed with this thin toner layer. This system is characterized by excellent dot reproducibility and effective production of uniform image with minimum irregularity. It is also considered to provide advantages in apparatus simplification, downsizing and cost-cutting. However, a heavy stress placed on the toner causes such a problem that the toner surface is degenerated, and toner or external additive agents are deposited on the surfaces of the toner regulating member or toner carrying member, with the result that fogging and contamination in the apparatus are caused both because of the poorly charged toner. As a result, the service life of the developing device will be reduced.
In the two-component developing system, on the other hand, toner is charged by triboelectric charging due to the mixture with a carrier. This is characterized by smaller stress and greater resistance to deterioration of toner. Further, as the carrier as a toner charging member has a greater surface area, it is relatively resistant to possible contamination by toner or external additive agents, whereby a longer service life can be expected.
However, when the two-component developer is utilized, contamination of the carrier surface is also caused by toner or external additive agents. The charge amount of toner will be reduced by a long-term use, and problems of fogging or toner splashing will arise. The service life is not sufficient, and prolonging the service life are desired.
One of the ways of prolonging the service life of the two-component developer is found in the Unexamined Japanese Patent Application Publication No. 59-100471, which discloses a developing device wherein a carrier is supplied little by little independently or together with toner, and the deteriorated developer of reduced charge is ejected in response to that, whereby the carrier is replaced by a new one, so that the percentage of the deteriorated carrier will be reduced. In this apparatus, the reduction in the toner charge due to carrier deterioration is kept to a predetermined level by replacement of the carrier. Thus, the service life is prolonged.
The Unexamined Japanese Patent Application Publication No. 2003-215855 discloses a two-component developer and a development method using the developer, the developer which is made up of the carrier and toner and is externally added opposite polarity particles having a polarity opposite. The opposite polarity particles in the development method act as abrasive powder and spacer particles, and are proved to have ability of reducing the deterioration of the carrier by the effect of removing the spent matters from the carrier surface.
The Unexamined Japanese Patent Application Publication No. H9-185247 discloses a so-called hybrid development method wherein the latent image on the image carrying member is developed using the toner carrying member that carries only toner from a two-component developer. The hybrid development method is characterized by the absence of irregularities on the image caused by a magnetic brush, and excellent dot reproducibility and image uniformity. Since there is no direct contact between the image carrying member and magnetic brush, there is no carrier movement to the image carrying member (carrier consumption). Thus, the hybrid development method has many advantages that cannot be expected in a conventional two-component developing system. In the hybrid development method, toner is charged by triboelectric charging with a carrier. The keeping of charge applying property is important for stabilizing the toner charging property and maintaining high image quality for a long period of time.
However, the Unexamined Japanese Patent Application Publication No. 59-100471 requires a mechanism to collect the ejected carrier and involves such problems as higher costs and environmental problems since the carrier is consumable. Further, as the printing operation in a predetermined amount must be done before the ratio of new carrier to old one is stabilized, initial characteristics cannot be kept for long. Further, in the Unexamined Japanese Patent Application Publication No. 2003-215855, when the area ratio of the image portion in the output image (image area ratio) is smaller, there is an increase in the amount of consumption of the opposite polarity particles on the non-image portion, as compared to the amount of consumption of the toner on the image portion on the image carrying member. When a great number of the images having a smaller image area ratio have been printed, there is a decrease in the amount of the opposite polarity particles in the developing device. Thus, the advantage of the opposite polarity particles for reducing the deterioration of the carrier cannot be fully demonstrated. This results in raising the problems of a reduction in the amount of toner charge and image deterioration. Further, the Unexamined Japanese Patent Application Publication No. H9-185247 has a problem that the carrier surface is contaminated by toner and post-processing agent as the volume of printing increases, with the result that the charge applying property of the carrier is deteriorated.